Thanks so much for getting in touch about this!
While it's possible that the charges are associated with a store who has included "SHOPIFY" in their customer statement descriptor, we can definitely check on our end to see if those descriptors match any account invoices. To do this, you can contact our support team via our Help Center here - select "Continue without logging in", enter a topic into the search field, then choose Get support. A support advisor will ask you to share a screenshot of the bank statement (obscuring any sensitive details), including an 8 or 9 digit invoice number. If it does turn out that your son has created a Shopify account, we can chat about our options regarding any charges, and give you a hand closing the account and removing the method of payment.
Please don't hesitate to let me know if you have any questions at all! Take care.